Previously, I’ve written about the craziness of the roundabout, otherwise known as the rotary interchange. More and more though, I find the rotary interchange to be one of the best designed traffic devices around, indeed, much better than the traffic light and the stop sign.

The advantages of the rotary are several. Firstly, no need to wait a fixed amount of time to go — since feed traffic always yields to rotary traffic, the amount of time to wait to enter the rotary dynamically adjusts to the density of traffic requiring interchange. Secondly, all traffic is treated fairly, once inside the interchange, regardless of routing destination. Thirdly, intersections become immediately generalizable to more than a four-way junction, as six-way, eight-way, or odd-number-way junctions are possible — indeed that’s primarily where rotaries are used in the first place. Fourthly, “special” paths like U-turns are no longer special, and so are allowed easily without problem.

In short, rotary interchanges make smart use of decentralized and self-organizing decision-making using simple local rules (i.e. yield to rotary traffic) to achieve complex results. Therefore it is genius.

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